Tube heater with flue gas recirculation and heating method



June 24, 1952 K. PERMANN 2,601,667

TUBE HEATER WITH FLUE GAS RECIRCULATION AND HEATING METHOD Filed Feb.12, 1948 5 Sheets-Sheet l INVENTOR KARL PERMANN BY W/v WM HIS ATTORNEYJune 1952 K. PERMANN 2,601,667

TUBE HEATER WITH FLUE GAS RECIRCULATION AND HEATING METHOD Filed Feb.12, 1948 3 Sheets-Sheet 2 INVENTOR: KARL PERMANN HIS ATTORNEY June 24,1952 K. PERMANN 2,601,667

TUBE HEATER WITH FLUE GAS RECIRCULATION AND HEATING METHOD Filed Feb.12, 1948 s Sheets-Sheet 5 36 ;E I1 I35 IOZ FIG. 3

IMVENTOR KARL PERMANN HIS ATTOR NEY TUBE HEATER WITH FLUE GAS RECIRCULA-TION AND HEATING METHOD Karl Permann, Oakland, Calif., assignor to ShellDevelopment Company, San Francisco, Calif.,

Patented June 24, 1952 a corporation of Delaware Application February12, 1948, Serial N 0. 7,987 15 Claims. (01. 122-275) This inventionrelates to improvements in combustion furnaces for heating fluids flowedthrough tubes, and to an improved method of heating such fluids, whereinthe heating tubes are arranged flue gas is withdrawn from the heater,insuch relation to the Venturi constriction as to facilitate regulationof the recirculated gas.

Still further objects of the invention will be within a heating space soas to cause hot com- 5 apparent from the following description, taken inbustion gases to flow longitudinally with respect connection with thedrawings forming a part of to the heating tubes, and wherein thecombustion this specification, wherein: ases are recirculated. Theheater and method Fig. 1 is a vertical sectional view of a preferredaccording to this invention are, for example, parembodiment of theinvention, parts being shown ticularly useful in the carrying out ofendothermic in el vati n; chemical reactions, such as thedehydrogenation Fig. 2 is a sectional view taken on line 2-2 of ofalcohols or other gaseous hydrocarbons which Fig. 1; takes place atabout 1000 F. Other examples are Fig. 3 is a vertical sectional viewshowing a the conversion of ethylene dichloride into vinyl modifiedconstruction; and chloride and hydrochloric acid at about 960 F" Fig. 4is a sectional view taken on line 4-4 of the cracking of hydrocarbonoil, and catalytic de- Fig, 3, hydrogenation processes. Briefly,according to the instant invention com- In heaters of this classdifficulty is frequently bustion gases are recirculated within theenexperienced because of uneven heating of the a closure of the heaterproper by means of the jet tubes, resulting in local overheating. It isaleffect of a gas, such as saturated or superheated ready known toprovide a more gentle hea steam, air, or combustion gases which havebeen by recirculating the flue gases, but this has recompressed. Iprefer to inject steam as the jet sulted in operating difficulties. Withthe heaters gas to avoid formation of an oxidizing atmosheretofore knownthe use of steam injectors for phere, and because it is readilyavailable in fureffecting circulation was not satisfactory because naceinstallations. However, the invention is not the primary andrecirculated combustion products were not always completely mixed anddifferent tubes were not heated equally.

It is an object of this invention to provide an improved tube heater andan improved method of heating flowing fluids wherein heat ofsubstantially uniform intensity is distributed among the several heatingtubes, and the tubes are heated substantially evenly alongtheir lengths.

It is a further object to provide an improved tube heater and animproved method of heating flowing fluids wherein recirculation ofcombustion products or flue gas is induced by the injection of a gas,such as steam, in a flow arrangement which will insure substantiallyeven distribution of heat to the several heating tubes. Ancillarythereto, it is anobject to provide an improved method of heating and animproved heater wherein the flue gas recirculation rate can be easilycontrolled.

Still another object is toprovide a heater having a combustion chamberfitted with one or more Venturi constrictions downstream from the burneror other source of combustion gases and having a nozzle for expandingand discharging a gas,

such as steam or air or combustion gas, into the.

Venturi throat in -'a manner to induce circulation of the flue gasthrough the combustion chamber, in combination with flow passage wayslimited to the use of steam and any gas, including air, may be used asthe jet gas. The jet gas is ejectedfrom one or more expanding nozzlesand directed as one or more jets through one or more Venturi throats,thereby entraining combustion gases. The hot combustion gases, from anysource, such as a liquid fuel burner, are commingled with therecirculated flue gas after the latter has passed in contact with theheating tubes and before flowing through the Venturi throat. The streamof hot combustion gases is therefore, successively augmented withrecirculated flue gas and with jet gas and, as a result, undergoes adecrease in temperature. The augmerited and partially cooled stream isthen passed through one or more elongated convection heating passagescontaining the heating tubes through which the fluid to be heated ispassed, the flow of the augmented gas stream being parallel to the axisof the heating tube and either concurrently or counter-currently to thedirection of flow of the fluid in the tubes. The reduced temperature andincreased volume of the augmented as stream minimize the temperaturegradient along the heating tubes and also equalize the heat transferrate from the gas to theheating tubes along the length and about thecircumferfor flowing theresulting augmented stream of ence of theheating 1illbescombustion gases in heat exchange with the heaty mymethod f mixing he fresh combusing tubes and to effect the recirculationof flue 131011 gases with the recirculated a s prior to gas through thecombustion chamber. Ancillary passage t h e Venturi throat (whereby thethereto, it is an object to arrange the discharge Venturi throat isdisposed upstream from the port, through which excess ornon-recirculated heating passages) insures a complete mixing of 3 thefresh and the recycled combustion gas and the jet gas; this results in amore uniform heating of the heating tubes.

Excess gases are withdrawn from the circulating gas stream at anydesired point in the circuit, and I prefer to withdraw them(approximately at a rate equal to the sum of the 'rate of admission offresh combustion gases and jet gas) just after the gas has traversed theheating tubes.

The method will be understood more completely from a consideration ofthe apparatus illustrated in the drawings, which illustrate certainpreferred embodiments of the invention, it being understood that theinvention is not limited to the specific arrangement of flow passagesshown therein.

Referring to Figs. 1 and 2, the heater is supported on a concretefoundation I having a central or inner wall la supporting a horizontalbulkhead l'b lined on its upper face with refractory fire brickssupporting a'plurality, for example, five burners '-2. These burnersreceive combustion air via opening lc. The wall la is continued upwardlyby a refractory brick section 3, a checker brick section 6, and arefractory section 5 having slots 5a near the top. The space above theburners 2 and within these sections is herein referred to as thecombustion space. The combustion space is fitted with a transversepartition or Venturi block 6 having a plurality, for example,five'perforations 6a forming Venturi throats. The lower portions ofthese throats may be lined with metallic sheathing as shown. Expansionnozzles 1 are mounted beneath each of the Venturi openings and disposedto discharge jet gas upwardly substantially tangent to the walls of theVenturi throats.

An outer furnace wall is provided surrounding the sections and '5 andspaced therefrom. This wall may be supported in any conventional manner. As shown in Figs. 1 and 2 it is suspended near a steel frameworkcomprising H--columns 8 resting on the foundation l and supportingvertical angle sections 9 through gusset plates ill. Thelan'gl'esections 9 support arcuate angles H from which fire resistant tileblocks 12 are sus pended by well known means, not shown, includingsuspension lugs. A sheathing of heat insulating material l3 surroundsthe blocks 12 and is also suspended from the angles l I Horizontal lb'eam's l4 extend across the top of the furnace, being ecured to theH-columns 8 and forming supports for'the suspended refractory roof I 5'and for'the heating tubes is and f1, suspended by collars Ida and l'la.These tubes extend vertically Within the annular connection heatingspace between the combustion chamber walls and the outer furnace wall,extending downwardly through openings in a lower, annular refractorywall l8, supported from the steel framework. The heating tubes aresealed to the roof [5- and lower wall 18 by means of refractory bushingsl9.

The lower portion of the outer wall has a series of openings 20communicating with an annular flue box 21 connected at one side to astack 22. A damper 23 is provided in the connection between the box 2|and stack.

The nozzles I are connected by a manifold 24 to' a steam line 25 whichis connected to a source of saturated or superheated steam, not shown.The valve 26 is open and the valve '2'! is closed. For superneating and/or generating the steam I may provide a steam coil 28 in the stack or inthe The tubes are 4 flue box 21; in this case the valve 26 is closed andthe valve 21 is open.

The fluid to beheated is/passed from inlet 29 to a manifold 30 connectedto the bottoms of the heating tubes 5. In this embodiment the heatingtubes are arranged to vaporize liquid and the "tubes Hi function aspreheaters and are of rela tively small diameter to effect theturbulence necessary for efficient heat transfer. The vapors pass intothe iiia'in heating tubes l'l via U-connector '31. The tubes I! are ofrelatively large diameter toacco'mmodate vapors and provide thenecessary space velocity for the particular use to which thefurnace isput. The fluid is discharged via radial pipes 32a to a circular exitheader 32.

'In theop'era-t'ion of the heater, hot combustion gases are 'g'enerated'by the burners 2 'andpassed upwardly through the combustion chamber,-radially 'outwardly'thi'oug h the slots 5a downwardly throughthe-annularheating space enclosing the heating tubes,and out through theflue box 21 and stack '22-. A portion of the flue gas is recirculat'edthrough the checkerbr-ic-k 4, augmenting the-sfiream-of combustion gasesand cooling the latter. circa-neon is induced by the action of thenozzles and the Venturi block- 6, wherein the augmented stream isentrained and still further'auginented by the 'jet gas. The pressure intheup'per portion-ofthe heater-above the Venturi block, may beatmespnene or slightly above atmespheric, while the pressure beneath theVenturi block may be somewhat less than atmospheri'c, although theinvention is not limited to operation at-such pressures. V

The recirculation rate is regulated by the rate o-f-flow o-f jet-gasissuing from the nozzles I and by the dam er 23. By recirculation rate Imean the weight ratio or the flue-gas flowing radially'i-n through thechecker br'iokto the cornbus-tio'n gases generated by the burners- 2.reeirculatiori rate' ma be varied, depending upon the desiredtemperature of the mixed gases flowing downwardly through the annularheating space. 12y way of exampn, andw-ithoutlimiting the inventionthereto, it'may' be stated that the circulation rate may in typicalinstallations be between about 1:0.5 and 1+3. Thus, when therecirculation rate is I12, the quantity of gas passed through theheating space is between 3 and 4 times as great as the quantitygenerated (consisting-of one part or fresh combustion gases, two partsof recirculated flue and a small amount, usually less than one part, ofjet gas).

Hea'tlosses are minimized in this heater by placing the hottest porn-eninside the inner, cylind'rical sections t 5 and by venting thee'xcessflue gas into the ride box 2-! at its lowest temperature. It is, furthendesirable toaveid- "ex-- cess combustion air in-order to improve thethermal efficiency and to avoid formation of an oxidizi ng gasfmi-xturewhich rapidly deteriorates the tubes and brickwork.-

The augme ted tream-or combustion gas is usually cooled lh' sowinthrough the Venturi throats because the jet-"gasis cooler than the gasstream entrained thereby.

Thecentral location or ar combustion-chamher and the recirculation offlue "gas make it possible to use less resistant walls for the outerwalls and minimizes heat lost by'r'adiatio'n. Thehating tubes are evenlyspaced in the annular heating space whereby the individual tubes atdilfrent'pbints' of the circumference of the heater receive anLemma-mount of heat. This axial symmetry, while particularly useful withThe the heater disclosed, is also useful in cases where the Venturithroat and jet nozzle 1' are omitted and the flow of flue gas iseffected by thermosyphon action or by fans or the like, and theinvention, in one aspect, resides in this arrangement of flow channels,regardless of whether means for inducing circulation are provided or ofthe type of such mean employed. I

An important feature of the invention is the arrangement whereby anaugmented stream of heating gas is flowed along the heating tubes at areduced temperature, whereby the temperature gradient lengthwise of thetube surface is reduced and the heat transfer rate in the direction ofthe length and circumference of the heatin tubes is nearly equalized.Another important feature resides in the location of the Venturi throatsimmediately downstream from the point of juncture of the fresh, hotcombustion gases and the recirculating flue gas. This arrangementinsures complete mixing of the fresh and recycled gases and the jet gasand results in the more uniform heating of the tubes.

By suspending the heating tubes from the top by collars resting on theexterior steel framework they are easily removed from the top, it beingonly necessary to disconnect them from the bottom manifolds. Byconnecting the preheating tubes I6 to the main heating tubes I1 at thetop.

wherein no vertical movement due to thermal expansion takes place, theprovision of fluid-tight connections are facilitated.

The heater may be modified in several respects. For example, thedischarge of excess flue gases may be effected from the top or from anintermediate level of the heating space, whereby the heating of thetubes may be varied to meet particular conditions. The outer walls maybe made entirely of refractory or of insulating material, instead ofcontaining a layer of each. Further, it is not necessary to arrange thecombustion chamber vertically or to place the heating tubes entirelyaround the combustion chamber; however, the arrangement shown ispreferred in that it provides a symmetrical arrangement and therebypromotes even distribution of heating gases. Finally, the suspension ofthe outer furnace wall and the tubes from a steel framework is not apart of the invention, and I may employ any other construc tion such as,for example, that illustrated in Figs. 3 and 4.

Referring to Figs. 3 and 4, the furnace comprises a concrete foundationIOI with a cylindrical portion IOIa lined with refractory fire bricksI03 and mm and insulating brickwork I33, and provided with a pluralityof premix burners I02 adapted to generate combustion gases by burning apreviously partly or fully mixed stream of fuel and air. Above thisbrickwork there are checker brickwork I04 and a cylindrical refractorysection I05 having slots I05a at the top. The section I05 has aplurality, e. f., five, radial fins I051) extending outwardly andsubdividing the annular heating space into a plurality of smallerheating spaces. A single Venturi block I06 having a throat I06a isfitted within the section I05 and provided with a plurality of nozzlesI01 for expanding jet gas into the Venturi throat. I v

The outer furnace wall comprises checker brickwork I 34 and acylindrical wall II2 which may be of suitable heat-resistant concrete orof refractory brick. Thefurnace top H5 rests on the wall H2 and iscovered by heat insulating material I35 and a steel plate I36. Heatingtubes II! extend vertically within the vertical heating spaces betweenthe walls: I05 and H2 and fins H151) and are suspended from the steelplateIISb by collars Illa. The tubes are connected at the top to amanifold I310 and at the bottom to a circular header I32 via radialconnectors I32a.

The flue exit duct is in the form of an annular flue box I2I incommunication with the heating space through the checker brickwork I34.It is connected to astack I22 through a passageway fitted with a damperI23. The nozzles I01 are supplied with jet gas by a manifold I24, pipeI25 and valve I26. The heater is operated in the same manner as thataccording to Figs. 1 and 2, except that no preheating tubes areprovided. The fluid to be heated may be fiowed either upwardly ordownwardly. through the heating tubes, depending upon the type ofheating desired. For example, in endothermic reactions, wherein theconversion temperature is to be maintained as uniformly as possible, itis usually important to avoid too great a difference in temperaturebetween the reacting gas mixture and the heating gas in the part of theheating tube where the reaction takes place, i. e., the outlet part ofthe tube. At the inlet end greater differences in temperature arepermissible Without causing any detrimental effect. In such a caseheating in concurrent flow with the heating gas is most suitable and thereaction mixture is flowed downwardly through the tubes Ill. For otherapplications of the heater countercurrent flow may, of course, be moreappropriate, and the mixture may be flowed upwardly through the heatingtubes.

I claim as my invention:

1. A tube heater comprising an enclosing wall structure forming acentral vertical combustion chamber, a source of hot combustion gasesnear the bottom of said combustion chamber, an outer wall spaced fromand surrounding said wall structure to form a vertically elongatedheating space therebetween, vertical convection heating tubes locatedentirely outside of said enclosing Wall structure and within saidheating space, passageways interconnecting the combustion chamber andthe heating space both at the top and at the bottom, means locatedwithin said combustion chamber for inducing upward movement ofcombustion gases and flue gas upwardly within said combustion chamber,whereby gases may circulate through said chamber and space, and a fluefor discharging flue gas.

2. The tube heater according to claim 1 wherein there are a plurality ofinterconnecting passageways at each end of the heating space and theheating tubes and the passageways are spaced substantially evenly aboutthe combustion chamber.

3. The tube heater according to claim 1 wherein the heating tubes arearranged in pairs, the

tubes ofeach pair being interconnected at the one vertical end of theheating space.

4. A tube heater comprising an enclosing wall structure forming acentral vertical combustion chamber, a source of hot combustion gasesnear the bottom of said combustion chamber, an outer wall spaced fromand surrounding said wall structure to form a vertically elongatedheating yspace therebetween, a plurality of radial partitions extendingfrom the enclosing wall structure to said outer wall and subdividingsaid eponoev heating space into a plurality of circumferentiallycontiguous vertical smaller heatingspaces, vertical convection heatingtubes locatederrtirely outside of said enclosing wall structure andwithin said smaller heating spaces, passageways interconnecting thecombustion chamber and the smaller heating spaces both at the top and atthe bottom, means located within saidvcombustion chamber for inducing.upward movement of combustion gases and flue gas upwardly within saidcombustion chamber, whereby gases may circulate through said chamberand-'smalier heating spaces, and a flue for discharging flue gas.

5. A tube heater comprisingan enclosing. Wall structure forming acombustion "chamber, a source of hot combustion gases near oneend of thechamber, a second wallstructureformi'ng, to?- gether with said firstwall-structure, ane'lon'gated convection heating space, convectionheating tubes within the heating space and locatedflto be shielded bysaid enclosing wallstructurefrom direct radiation from the said sourceof "combustion gases extending longitudinally with respect to theheating space, passageways interconnecting the ends of said heatingspace with spaced points in said combustion chamber, a Venturi con-'-striction within said combustion chamber inter mediate said spacedpoints. nozzle means. for discharging a fluid jet into the throat of:said Venturi constriction, and a flue for discharging flue gas.

6. A tube heater comprising a first inner wall of refractory materialforming a vertical com-- bustion chamber, a source of hot combustiongases near the bottom of said combustion chamher, a second outer wall ofinsulating ref-ractory material surrounding said first wall and spacedtherefrom to form a vertically elongated heating space therebetween,vertical convection heating tubes located entirely outside of said firstinner wall and within said heating space, passageways interconnectingthe ends of said heating space with spaced points in said combustionchain ber above said source of combustion gases a Venturi constrictionwithin said combustion :chamber intermediate said spaced pointsnozz-lemeans for discharging a fluid :jet upwardly into the throat of saidVenturi constriction, and. a1 flue for discharging. flue gas.

'7. The tube heater according toclaimfi wherein the flue is connected tothe bottom of= said heating space. I

8. The tube heater according to claimfi wherein there are a plurality ofpassageways atv each end of the heating space, the heating tubes andpassageways are spaced substantially evenly about the combustion chamberandthe-flue comprises an annular flue box surrounding the-outer wall andin communication with said heating space through a plurality ofpassageways. disposed circumferentially with respect-totheheating space.

9. A tube heater comprising. an inner wa-li structure of refractorymaterial form-inga vertical combustion chambensaidwallstructurecomprising an open checkerwork section near the bottom anda solid section above said checkerwork section, a plurality ofcircumferentially spaced openings at the top of said solid: section, aVenturi block withinsaid solid section. having one or more Venturi.constrictions, a source of hot combustion gases'beneathsaid.openrcheclrer work section, an outer wauoi insulating one fractorymaterial surrounding said inner wall and spaced therefrom to form avertically elongated heating space therebetween, said space being incommunication with the combustion chamber through said open checkerworksec tion and "said openings, a plurality of vertical convection heatingtubes Within said heating space spaced substantiallyevenly about thecombustion chamber, a second, outer wall structure surrounding. saidinner wall and forming a flue box, passage-ways interconnecting saidheating spa-ceand said flue box at a plurality of points, av flue fordischarging flue gas from said flue box, and a damper for controllingthe discharge of the flue gas. 1

10. The heater according to claim 9 wherein the flue box is located nearthe bottom of the heating space.

11. A method of heating flowing media comprising'flowing the medium tobe heated through a: tube, flowing an augmented stream of combustiongases, described hereafter, through an elongated convection heatingspace longitudinally with respect to said tube, thereafter admixing atleast a portion of the combustion gases with fresh, hot combustiongases, to form an augmented stream of combustion gases, forcing flow ofthe augmented stream toward said convection space through the entrainingaction of a jet, thereby establishing a circulating stream of combustiongases, and removing a portion of said circulating stream. V

12. The method according to claim 11 wherein the portion of thecirculating stream is removed from the. augmented stream immediatelyafter passage thereof. through the convection space;

13. The methoda'ccording to claimll wherein the flow oftheaugmentedstream is induced by injecting a jet of a gas intoa Venturiconstriction.

:14,- A method: of. heating flowing media comprising flowing-the mediumto be heated through a plurality of vertical tubes, flowing anaugmerited: stream of combustion gases, described hereafter, downwardlythroughfa vertically elongated convectionheating space along saidheating tubes, thereafter admixing a portion of the combustion gaseswith fresh, hot combustion gases, to form an augmented stream" :oicombustion gases, inducing upward flow oi the aug merited streainexternally of said convection space toward the top of said convectionspace by means o'fthe entraining action 'of a jet, thereby establishinga circulating stream of combustion gases, andreihoving a portion of saidcirculat ingstream.

I 15. The-method according'to claim 14 wherein the upward flow'oftheaugmented stream is inducedbyinjectingja jet of steam-into a Venturiconstriction. 7

PERM'ANN.

Resonances CITED The fol-lowingi'eference'sare of record: in: the file"of thispatentt' UNITED STATES PATENTS

